fix: significantly reduce rust toolchain size

[savonarola]

Currently:

~/.rustup/toolchains/1.88.0-x86_64-unknown-linux-gnu >du -hs .
1.3G    .
~/.rustup/toolchains/1.88.0-x86_64-unknown-linux-gnu >find . -type f | wc -l
47233

This also unconditionally slows down each application test run by over 2 mins

because run.sh chowns these 47k files https://github.com/emqx/emqx/blob/8f3adee478017018f18f9d177c5d4ac0f6ba59e0/scripts/ct/run.sh#L329

With minimal profile:

~/.rustup/toolchains/1.88.0-x86_64-unknown-linux-gnu >du -hs .
551M    .
~/.rustup/toolchains/1.88.0-x86_64-unknown-linux-gnu >find . -type f | wc -l
125